1. Field of the Invention
The present invention relates to the field of current and voltage sources and references, and particularly to those that are proportional to absolute temperature
2. Prior Art
In general, the base emitter voltage of a junction transistor is given by the equation:       V    BE    =                    V        go            ⁢              (                  1          -                      T                          T              0                                      )              +                  V        BEO            ⁢              (                  T                      T            0                          )              +                  nKT        q            ⁢      ln      ⁢              xe2x80x83            ⁢              (                              T            0                    T                )              +                  KT        q            ⁢              ln        ⁢                  (                                    J              C                                      J              C0                                )                    
where:
T=temperature
TO=an arbitrary reference or starting temperature
JC=the transistor collector current density
JCO=collector current density for which VBEO was determined
Vgo=semiconductor bandgap voltage extrapolated to a temperature of absolute zero
VBEO=base to emitter voltage V at TO and ICO
q=electron charge
n=structure factor
K=Boltzmann""s constant
If one subtracts the VBEs of two transistors a and b operating with different current densities, such as two identical transistors a and b operating with different collector currents, or two transistors a and b of different areas but otherwise identical and operating with equal collector currents, there results:             V      BEa        -          V      BEb        =                    KT        q            ⁢      ln      ⁢              (                              J            Qa                                J            C0                          )              -                  KT        q            ⁢      ln      ⁢              xe2x80x83            ⁢              (                              J            Qb                                J            C0                          )            
or:             V      BEa        -          V      BEb        =            KT      q        ⁢    ln    ⁢          xe2x80x83        ⁢          (                        J          Qa                          J          Qb                    )      
Where: JQa and JQb are the current densities in transistors a and b, respectively
Thus, the difference in the VBEs of the two transistors operating with different current densities is proportional to absolute temperature (PTAT).
Various references that use the foregoing principle to provide an output proportional to absolute temperature are well known. However the output of such references also has an undesired dependence on the inputs to the references. Since such references are frequently used as sources for bias currents in linear circuits, this dependence can increase noise and create other undesired effects in the circuits in which they are used.
Proportional to absolute temperature references having reduced input sensitivity are disclosed. The references utilize four bipolar transistors, at least one of which is of a different size, coupled to a resistor in a loop, whereby the difference in the VBEs of the transistors appears as a voltage across the resistor. The addition of a further resistor of a selected size in the base circuit of one of the four transistors provides an input variation of an opposite sign to that caused by the finite base currents of the transistors, thereby substantially reducing the input voltage (current) dependence of the proportional to absolute temperature references. Various embodiments are disclosed.